In a small saucepan, heat butter, chocolate and water until chocolate melts and mixture is smooth, stirring constantly. Set aside to cool.

Sift together into a large bowl the flour, sugar and baking soda. In a small bowl, beat the egg slightly and combine with sour cream and vanilla. Stir into flour mixture. Pour in the cooled chocolate mixture. Beat on low speed for 5 minutes until completely combined and the consistency of heavy cream.

Pour batter into prepared 9 inch tube pan and bake 45-60 minutes, or a toothpick inserted in the center comes out clean. Let cool in pan for 10 minutes, then remove and cool completely.